We selected 22 patients who reported exacerbations of asthma symptoms or rhinitis in only one particular dwelling and were free of symptoms in another. These aggravations were observed several times, suggesting house dust-induced asthma. The 22 patients (12 women and 10 men; mean age, 25.4 ? 9.7 years) were monosensitized to Dermatophagoides pteronyssinus; they had a positive intradermal test result to D. pteronyssinus after injection of 0.04 ml of a lo-’ mg/ml D. pteronyssinus extract (Institut Pasteur, Paris, France) and a Phadebas Radio Allergo Sorbent Test (RAST, Pharmacia AB, Uppsala, Sweden) for D. pteronyssinus score of equal to or greater than 3. For each patient, two house dust samples were obtained. The first one was from the dwelling in which they were free of symptoms, and the second one was from the dwelling in which they had symptoms of asthma or rhinitis. The 44 house dust samples were collected by the patients, who vacuumed their mattresses for 2 minutes per square meter. Dust bags were stored at 4” C in sealed plastic bags. Semiquantitative guanine determination was performed with the Acarex-test (Allergopharma, Hamburg, Germany). According to the manufacturer, results are semiquantitatively expressed in four classes, for which the guanine content is indicated as 0, +, + +, or + + +. A Wilcoxon signed-rank test was performed to determine whether there was a significant difference between the Acarex-test results from the dwellings in which patients were free From “Service de Pneumologie, Hopital Civil, Hdpitaux Universitaires de Strasbourg; and %he Laboratoire d’EpidCmiologie et de Sante Publique, FacultC de Mtdecine, Strasbourg.